Field of the Invention
The invention relates to a polyaxial bone anchoring device for anchoring a stabilization rod to bones or vertebrae. The bone anchoring device includes an anchoring element, a receiving part for receiving a head of the bone anchoring element and for receiving a stabilization rod to be connected to the anchoring element. The anchoring element is pivotably held in a seat in the receiving part, and can be fixed at an angle relative to the receiving part by exerting pressure onto the head via a pressure element which is arranged in the receiving part. The anchoring element may have a head with a spherical outer surface portion including a largest outer diameter, and the pressure element may include a head contacting surface portion including at least two circumferentially distinct lugs extending along a region including the largest outer diameter of the head when the pressure element is placed onto the head. The pressure element may have an undersize or be undersized in a region of the lugs corresponding to the head, such that the head is clamped by friction between the head and the lugs, causing the anchoring element to be maintained at a desired temporary angular position before final locking of the head.
Description of Related Art
US 2004/0267264 A1 describes a polyaxial fixation device wherein the polyaxial bone screw includes an engagement member that is adapted to provide sufficient friction between the spherical head and the receiver member to enable the shank to be maintained in a desired angular orientation before locking the spherical head within the receiver member. The engagement member is realized, for example, by an open snap ring around the head or by spring members provided at the compression cap to frictionally engage the spherical head or by a slot provided in the compression cap.